This invention relates to an electron gun of a cathode ray tube, more particularly a cathode system (that is, a cathode electrode and a first grid electrode) which can produce a small electron beam spot.
The potential distribution and the path of electron beams near the cathode electrode of an electron gun utilized in a cathode ray tube are generally shown by FIG. 1. Although it is difficult to strictly show the behavior of the electrons near the cathode electrode, the electrons 3 emitted from the surface 2 of the cathode electrode 1 are subjected to a force acting in a direction perpendicular to equipotential surfaces 4 so that the electrons form a crossover 5. More particularly, in an electron gun utilized in an ordinary cathode ray tube, the electric field in front of the cathode electrode not only accelerates the electrons but also forms an electron lens. However, due to the aberration of the electron lens, there is a limit for the minimum diameter of the crossover. Generally, unless the crossover diameter is minimized, when the electron gun is used in a cathode ray tube, it is difficult to make small the spot diameter on the fluorescent screen. As shown in FIG. 1, in an ordinary cathode ray tube, there are also provided a first grid electrode 6 applied with a predetermined potential, for example -10 V, for forming an electron lens near its opening 7 (iris) and a second grid electrode 8 supplied with a predetermined potential, for example +500 V, with respect to the cathode electrode for accelerating the electrons emitted from the cathode electrode. Of course, the second grid electrode 8 too is provided with an opening 9 for passing the electron beams. The cathode electrode 1, and the first and second grid electrodes 6 and 8 are spaced from each other and coaxially arranged along the axis of an evacuated envelope, not shown, of the cathode ray tube.
For the purpose of removing such limit, it has been proposed to form a cathode surface 2 as a portion of a cylindrical or spherical surface as shown in FIG. 2 so as to form a cylindrical or spherical potential distribution thereby readily focusing the electron beam 3 generated from the cathode surface 2. In FIG. 2, numeral 6 represents a primary grid electrode concentric with the cathode surface, O the center of the sphere and R the radius thereof. A typical example of the electron gun having the construction described above is the so-called Pierce type electron gun. Different from the cathode system utilizing a flat electrode as the cathode surface, in the cathode system having a cathode surface in the form of a portion of cylindrical or spherical surface, it is necessary to always correctly align the axis of the cathode electrode 1 and the center of the primary grid electrode 6, necessitating troublesome assembling. For this reason, the cathode system of this construction has not been used extensively.